HVAC service personnel must measure the system refrigerant pressure in order to install or service a HVAC system. This measurement is accomplished with an analog pressure gauge. Typically, the gauges are mechanical and contain a curved tube, which bends in response to the applied pressure. Connected to the tube are series of watch-like gears that connect and rotate the indicating needle located on the front of the gauge.
Gauges can either be permanently attached to the equipment or incorporated into portable tools or manifolds. Normally, a manifold or gauge manifold holds two gauges for pressure measurement and includes valves for installation or removal of the refrigerant from the HVAC system.
Typically, two gauges with different pressure ranges are used for the basic pressure measurements of the refrigerant. The main analog scale of each gauge indicates pressure in PSI (pounds per square inch) or KPa (metric Kilo-Pascals). Additional inner analog scales are also printed on the face of the gauge. The additional inner scales (circular bands of numbers) indicate the saturated vapor equivalent temperature for different refrigerants. Because of the limited space on the gauge face, only two or three different inner scales for different refrigerants can fit onto the face of any one gauge. FIG. 1 shows a typical prior art pressure gauge.
The saturated vapor equivalent temperature scale of the gauge is of importance to the technician because the saturated vapor equivalent temperature indicated on the gauge for the particular refrigerant is used to ascertain the temperatures in parts of the system during charging, servicing, or monitoring. The correct inner gauge scale must be matched to the type of refrigerant in the system. If the gauge does not have an analog scale that matches the refrigerant in the system being serviced, the technician must consult a table that converts the pressure read on the gauge to the saturated vapor equivalent temperature. Because HVAC systems respond very slowly and the pressure and the saturated vapor equivalent temperature must be continuously monitored during refrigerant charging, the use of a lookup chart is inconvenient, time consuming, and error prone.